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Abstract

Three types of Co3O4 nanoparticles are synthesized and characterized as a catalyst for the air electrode
of a Li/air battery. The shape and size of the nanoparticles are observed using scanning
electron microscopy and transmission electron microscopy analyses. The formation of
the Co3O4 phase is confirmed by X-ray diffraction. The electrochemical property of the air electrodes
containing Co3O4 nanoparticles is significantly associated with the shape and size of the nanoparticles.
It appears that the capacity of electrodes containing villiform-type Co3O4 nanoparticles is superior to that of electrodes containing cube- and flower-type Co3O4 nanoparticles. This is probably due to the sufficient pore spaces of the villiform-type
Co3O4 nanoparticles.

Keywords:

Introduction

A significant increase in the energy density of rechargeable batteries is required
to satisfy the demands of vehicular applications and energy storage systems. One approach
to solving this problem is the introduction of a new battery system having a higher
energy density. Li/air batteries are potential candidates for advanced energy storage
systems because of their high storage capability [1-3]. They do not store a 'cathode' in the system, which allows for a higher energy density
than any other commercial rechargeable batteries. Instead, oxygen from the environment
is reduced by a catalytic surface inside the air electrode. Thus, catalysts are key
materials that affect the capacity, cycle life, and rate capability of such batteries.

In this study, the Co3O4 nanoparticles of various shapes and structures were tested as catalysts of air electrodes
for rechargeable Li/air batteries. Co3O4 with a spinel structure has attracted a considerable interest as a potential catalyst
in various application fields [4-7]. In particular, this study was motivated by the notion that the catalytic efficiency
of oxides is highly dependent on their morphology, size, and crystal structure [8,9]. Herein, three types of Co3O4 of various shapes and morphologies were synthesized, and the electrochemical properties
of the air electrodes containing Co3O4 nanoparticles were characterized.

Experimental details

Three types of Co3O4 nanoparticles were prepared by a hydrothermal reaction using cobalt nitrate (cube
type, flower type) and cobalt chloride (villiform type), considering previous reports
[10,11]. Surfactants such as urea were also added to obtain nanosized particles. X-ray diffraction
[XRD] patterns of powders were measured using a Rigaku X-ray diffractometer (Rigaku
Corporation, Tokyo, Japan). The microstructure of the powder was observed by field-emission
scanning electron microscopy [FE-SEM] (JEOL-JSM 6500F, JEOL Ltd., Akishima, Tokyo,
Japan) and field-emission transmission electron microscopy [FE-TEM] (JEOL-JEM 2100F
JEOL Ltd., Akishima, Tokyo, Japan). The electrochemical performance of the air electrode
containing Co3O4 nanoparticles was examined using a modified Swagelok cell, consisting of a cathode,
a metallic lithium anode, a glass fiber separator, and an electrolyte of 1 M LiTFSI
in EC/PC (1:1 vol.%). The cathode contained carbon (Ketjen black EC600JD, Akzo Nobel,
Amsterdam, The Netherlands; approximately 1420 m2·g-1), catalysts (Co3O4 nanoparticles), and a binder (PVDF; Sigma-Aldrich, St. Louis, MO, USA). The molar
ratio of carbon to catalysts was adjusted to 95:5. The binder accounted for 20 wt.%
of the total electrode. The cells were assembled in an Ar-filled glove box and subjected
to galvanostatic cycling using a WonATech (WBCS 3000, Seocho-gu, Seoul, Korea) charge-discharge
system. Experiments were carried out in 1 atm of O2 using an air chamber.

Results and discussion

Scanning electron microscopy [SEM] and transmission electron microscopy [TEM] were
employed to investigate the shapes of the samples (Figure 1). Cube-type Co3O4 nanoparticles have a homogeneous cubic morphology (Figure 1a). The length of the nanocube was around 200 nm, and the dominant exposed plane of
the cube-type Co3O4 seemed to be {001}. The villiform-type Co3O4 particles were formed by a nucleus covered with numerous micrometer-sized nanorods.
In comparison with the length, the diameter of the nanorod was very small (less than
100 nm). It is interesting that the villiform-type Co3O4 has a rough surface. As shown in the TEM image (Figure 1b), the nanorods seemed to be stacked with smaller nanoparticles with a diameter of
approximately 80 nm. The flower-type Co3O4 seemed to have a similar shape and size to those of the villiform-type Co3O4. However, the nanorods of the flower-type Co3O4 had a sharper end, smoother surface, and smaller diameter than those of the villiform-type
Co3O4. Moreover, in contrast with the villiform-type Co3O4, the nanorods of the flower-type Co3O4 particles were almost separated during the preparation process for the TEM experiments
(Figure 1c). This implies that the flower-type Co3O4 particles may turn to the nanorod type during the electrode fabrication process because
of vigorous mixing in making a slurry. The crystallinity of the three types of Co3O4 nanoparticles was investigated by XRD. As shown in Figure 2, all XRD peaks of the cube-type Co3O4 nanoparticles can be indexed to the Co3O4 spinel phase, indicating a single-phase sample. Most diffraction peaks for villiform-
and flower-type Co3O4 particles were also identical to those of the typical Co3O4 phase; however, small impurities could be detected in the diffraction patterns.

The electrochemical properties of the air electrodes containing Co3O4 nanoparticles were characterized at a constant current density of 0.4 mA·cm-2 at 30°C. Figure 3a shows the initial voltage profile of the electrodes containing the Co3O4 nanoparticles in the voltage range of 4.35 to 2.3 V. The discharge capacity shown
in Figure 3 is based on the weight of carbon (Ketjen black) in the air electrode, which has generally
been used for expressing the capacity of an air electrode [1,8,9,12]. The average charge and discharge voltages of the air electrode containing the Co3O4 nanoparticles were approximately 4.2 and 2.6 V, respectively. The initial discharge
capacity of the electrode was highly dependent upon the type of Co3O4 nanoparticles. The electrode containing villiform-type Co3O4 nanoparticles showed a relatively higher initial discharge capacity (approximately
2, 900 mA h·g-1) than with the other electrodes. In contrast, the initial discharge capacities of
the electrodes containing flower-type Co3O4 nanoparticles were just about 1, 800 mA h·g-1 although they have a shape very similar to the villiform-type Co3O4 nanoparticles. As shown in Figure 3b, the cyclic performance of the air electrodes was not satisfactory. Actually, capacity
fading has been a typical feature of all previous results about air electrodes [8,12,13]. It has been known that cycle degradation is associated with irreversible reaction
products, which accumulate in the pores of the electrode at a discharged state [13,14]. It seems that the practical rechargeability of air electrodes has yet to be achieved
before these can be put to practical use.

After 10 cycles, the electrode was discharged to 2.3 V, and the surface was observed
by SEM to investigate the morphology change during cycling. In the SEM images of the
air electrodes before testing, the Co3O4 nanoparticles and carbon (Ketjen black) could be clearly identified (Figure 4). It was noticeable that the villiform-type Co3O4 nanoparticles maintained their shape during the electrode-fabrication process. However,
the flower-type Co3O4 nanoparticles were almost separated to become the nanorod type. When they discharged
to 2.3 V, it was observed that the surface of the electrode was homogenously covered
with precipitates, which appeared to be reaction products such as lithium oxides,
and lithium carbonates formed due to electrolyte decomposition [15,16]. These reaction precipitates could block the catalyst/carbon contact area, thereby
preventing O2 intake and Li+ delivery to the active reaction site and terminating the discharge process. According
to previous reports [13,14], there was a strong correlation between average pore diameter and discharge capacity.
Reaction precipitates are likely to be formed near active sites so that the micropore
of a porous electrode would be easily sealed with precipitates of lithium oxides during
discharge. Thus, securing enough space between catalytic active sites might increase
the discharge capacity of the air electrode. The cube- and flower- (nanorod- in the
electrode) type Co3O4 nanoparticles may be well covered with small carbon particles (Ketjen black) in the
air electrode so that a sufficiently small pore space could be obtained. On the other
hand, the villiform-type Co3O4 nanoparticles were composed of a nucleus covered with many nanorods of approximately
100 nm in size, which could offer enough space between active catalytic sites. Thus,
a greater amount of lithium oxide precipitation may be needed to block the pore orifices
and terminate the discharge process; this could be an explanation for the higher discharge
capacity of the air electrode containing villiform-type Co3O4 nanoparticles in comparison with the air electrode containing other types Co3O4 nanoparticles.

Figure 4.SEM images of the air electrodes. Air electrodes composed of Co3O4 nanoparticles, carbon (Ketjen black), and binder before the test and after discharge
at 2.3 V. (a) Cube type, (b) villiform type, and (c) flower type.

Conclusions

Cube-, flower-, and villiform-type Co3O4 nanoparticles were synthesized and introduced as catalysts for Li/air batteries. The
electrochemical properties of the air electrodes containing Co3O4 nanoparticles were found to be highly dependent on the type of Co3O4 nanoparticles. The electrode containing villiform-type Co3O4 nanoparticles showed a higher discharge capacity than the electrodes containing other
types of Co3O4 nanoparticles. This is likely due to the relatively sufficient pore space between
active catalytic sites, which stores a large amount of reaction products.